This invention relates to a flow indicating device and, more specifically, it relates to a rotary sight flow indicator with an internal shroud covering a portion of the rotor.
In-line sight flow indicators offer a simple, inexpensive means of monitoring flow rate and direction, as well as the color and clarity of the fluid. Sight flow indicators are used to provide a reliable verification of flow in filter, lubrication, and cooling lines and to provide a positive and reliable backup for meters, switches, process indicators, and other control devices. Since sight flow indicators provide a visualization of flow they are readily interpreted by the observer. A number of different sight flow indicators have been developed to serve specific needs. The rotary sight flow indicator has been found to be the best way to show flow in opaque fluids because of the high visibility of the rotor. The rotary sight flow indicator is also preferred in certain applications because the motion of the rotor is visible from a distance.
Heretofore, rotary sight flow indicators have been constructed with a flow diverting element at the inlet of the indicator to direct flow to one side of the cavity containing the rotor in order to cause the rotor to turn. The flow diverting element functions by creating a partial blockage of the inlet to the cavity such that the flow impings on only a fraction of the rotor blades. The asymmetric flow impingement on the rotor results in an unbalanced torque which causes the rotor to turn such that rotor blades upon which the flow is impinging move in the direction of the flow. This movement of the rotor blades, which can be visually observed through a view port, provides a positive indication of flow in the fluid. In a flow indicator capable of displaying flow in either direction (bi-directional), two such flow diverting elements are required, one at either end of the cavity.
There are several disadvantages to the design of conventional rotary sight flow indicators. The most undesirable characteristic is the high pressure drop arising from the flow diverting elements which introduce a large resistance to flow by decreasing the flow area at the inlet to the cavity. High pressure drop translates to high energy consumption and high operating costs. In fact, this high pressure drop is the main reason why rotary flow indicators are considered less appropriate than other types of flow indicators in many applications, in spite of the fact that rotary flow indicators provide a better visual flow indication. Another disadvantage of the conventional rotary flow indicators is their insensitivity at low flows. In order to enhance the low flow sensitivity of a convention sight flow indicator a strong flow deflection is required, which results in an increased pressure drop at elevated flows. Even with the penalty of increased pressure drop the conventional design often fails to provide the desired flow sensitivity at low flows. As a result, the flow required to turn the indicator rotor is too high for many applications. In summary, the conventional rotary sight flow indicator causes far to much flow resistance, particularly in the bi-directional configuration, and does not have good low flow sensitivity.